hardening after they've been made and since I haven't yet built a decent
toolpost grinder. I thought I'd try an experiment to see how much distortion I
might get when hardening the steel that I'm using to make the gears and shafts
from.
I'd read, via Tubal Cain and others, that salt baths can be very handy things
to heat steel up in. They should be less likely to over/under heat bits of
the part you are heating than an open torch and you shouldn't get the scaling
problems you would in an open atmosphere.
The steel I'm using is EN24 aka AISI 4340. This steel is hardened by
austenitizing at 802 Centigrade and quenching in oil and (in my case)
tempering at 200 centigrade.
It just so happens that common table salt melts at 801 centigrade.
Armed with this knowledge and the fact that SWMBO had over-ordered on salt
when making a batch of play-dough for some kids lead to today's experiment.
I measured the trial part as well as I could and noted the dimensions on the
drawing I had made it from. Then set up a crucible on some fire bricks and
surrounded it with the rockwool that I've used in the past for heating things
in. A pound or so of salt was poured into the dried out crucible and set to
heat. While that was warming up I set up a paint kettle of new 15W-40 motor
oil on a gas ring for the purposes of quenching to the desired temperature.
I was rather worried that the oil was approaching its flash point and only
heated it to 170 centigrade instead of the 200 that I had wanted... must get
some heavier oil.
The pictures are here:-
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I quenched the part, then hurried indoors to put it in the pre-heated kitchen
oven for half an hour or so, then I let it cool. Once I'd cleaned off the
sticky oil blacking and the remaining specks of salt, I re-measured it. It
looks like the biggest change in dimension is of the order of about a tenth of
a thou per inch. I can live with this with no problems at all.
Having a pot fill of red hot liquid with nothing to stop it from tipping over
is a little bit scary. I think that a properly built salt bath is definitely
there in the future. I was very impressed by the complete lack of scale on the
steel.
I don't yet know whether I achieved the hardness that I was aiming for. The
part isn't file-hard, but is very hard. This is what I was expecting, since I
was aiming at about 42HRC. I'll see if I can get it tested at work tomorrow to
put my mind at rest.
It warmed the shed up as well :-)
I must put a heat spreader under the firebricks before I do the next one
though. I ended up with the poor old workmate smouldering before I'd finished.
Mark Rand
RTFM